German Patent Specification DE-PS 33 16 595 (corresponding to U.S. Pat. No. 4,551,302) discloses palladium alloys for producing fixed and removable dentures. The disclosure of this patent specification is incorporated herein by reference. The alloys described in that specification contain 65 to 85% palladium, 0 to 10% gold and/or 0 to 5% platinum, 0.1 to 10% tin, 1 to 10% gallium, 1 to 12% copper as well as 0.05 to 1.5% ruthenium and/or 0.05 to 0.7% rhenium. Thus, the base metal content can be up to 32% by weight.
When palladium alloys with such high base metal contents are melted in an oxidizing atmosphere, they occasionally tend to form a stable oxide film which floats on the liquid melt. The formation of such an oxide film brings about the accumulation and combustion of oxidizable alloy constituents. If the alloy is remelted several times, it can result in an undesired change of the alloy composition. Moreover, when a dental technician melts such alloy in an oxidizing atmosphere, he may not be able to see through the surface oxide layer and this is not able to perceive whether the alloy located under the oxide layer is sufficiently liquefied to be cast.
Another undesired phenomenon associated with the formation of an oxide film is the fact that the melting crucible occasionally does not discharge completely, so that slag residues remain on the crucible bottom. These oxide films occur in particular in the case of alloys which contain more than 10% base metals, depending on the type of base metals added.